1. Field of the Invention
This invention relates generally to an image compression technique, and more particularly to an image compression technique that operates on blocks of pixels using a block-based coding scheme that codes different types of image data in different ways based on parameters that are determined by the coding process itself. Still more particularly, the image compression technique of the present invention evaluates the image to be compressed to determine its make-up and uses this information to determine the most effective combination of coding techniques to use to guarantee a desired compression ratio.
2. Description of the Related Art
A typical high quality digitized color image may use 24 bits per pixel (bpp)--8 bits for each of the three basic color components: red (R), green (G) and blue (B) in RGB color space or for each of the three basic luminance-chrominance components: luminance (Y), chrominance (C.sub.b) and chrominance (C.sub.r) in YC.sub.b C.sub.r color space. In the uncompressed state (i.e., in the spatial or pixel domain), such images are simply too costly and time consuming to transmit and store. The high transmission time and memory requirements for high quality color images is apparent when compared to gray-scale images that may use 8 bpp or bi-level images that use only 1 bpp. Thus, applications and devices which store or transmit high quality digitized color images typically do so in a compressed format, using one of the currently available compression techniques.
Various image compression techniques have been proposed to reduce the number of bits used to represent a digitized color image while, at the same time, providing quality image representation. These techniques generally seek to strike a balance between transmission time and memory requirements on the one hand and image quality on the other. Some of these techniques are "lossless," meaning that they preserve all information of the original image so that it is reproduced exactly when the data is decompressed. Other techniques, commonly referred to as "lossy," discard information which is visually insignificant. By only approximating the original image (rather than reproducing it exactly), lossy techniques are generally able to produce higher compression ratios than lossless techniques. In selecting the appropriate compression technique among those currently available, the user must consider the particular image to be compressed, the desired compression ratio and image quality as well as transmission time and memory requirements, with the understanding that higher compression ratios lead to lower transmission times and memory requirements but also produce lower quality images.
One of the problems with the currently available image compression techniques is that most tend to be designed for one type of data and generally do not work well on hybrid color images (that is, images containing text, graphics, as well as synthetic and natural images). Since different types of data have different frequency characteristics, it is difficult to achieve a high compression ratio by applying a single coding mode to a hybrid image without sacrificing image quality. In order to effectively compress hybrid images an adaptive coding technique is needed.
One such adaptive coding technique is proposed in U.S. Pat. No. 5,696,842 which provides a coding process that separates a document image into blocks and classifies them as "picture" blocks or "black-and-white" blocks using a block classification algorithm that employs a complex edge-detection mechanism. The blocks are then coded according to their classification. Arithmetic coding is used for "black-and-white" blocks and ADCT for "picture" blocks. While this coding system offers certain advantages over non-adaptive coding systems, it has certain disadvantages as well. For example, the block classification scheme is relatively complex and is not tied to the coding process itself, which makes for a relatively high overhead requirement. In addition, the adaptive coding technique of this patent does not offer guaranteed compression rate control.